1. Field of the Invention
The present invention relates to a substrate processing method.
2. Description of the Related Art
Examples of a processing method for forming a through-hole in a substrate include a method that uses reactive ion etching, which is a type of dry etching. A substrate processing method that uses reactive ion etching, which is a method of forming a through-hole by using an etching gas, is suitable for forming a vertical through-hole. In particular, reactive ion etching may be used to form a liquid supply port (through-hole) in a substrate of a liquid ejection head, typically an inkjet head. A vertical hole can be easily formed by reactive ion etching. In other words, a through-hole formed by reactive ion etching is not enlarged horizontally. Therefore, the size of a substrate can be reduced.
In order to form a through-hole in a substrate by reactive ion etching, a layer made of an insulator (etching-stop layer) is formed on a first surface side of the substrate, and reactive ion etching is performed from a second surface that is on the opposite side to the first surface, until the reactive ion etching reaches the etching-stop layer. The etching is finished when the etching reaches the first surface and the etching-stop layer. Then, the etching-stop layer is removed, thereby forming a through-hole in the substrate.
With such a substrate processing method, a phenomenon called “notching” may occur when reactive ion etching reaches the etching-stop layer. The term “notching” refers a phenomenon in which a through-hole is enlarged in a direction perpendicular to the direction in which the through-hole is to be formed due to the effect of electric charging. The problem of notching will be explained by using an example in which a liquid supply port (through-hole) is formed in a substrate of a liquid ejection head. After etching of the substrate from the second surface side has reached the etching-stop layer on the first surface side, the etching progresses along the etching-stop layer. In other words, the etching progresses in a direction perpendicular to the direction in which the supply port is to be formed. If notching occurs, the opening of the through-hole on the first surface side is enlarged to an undesirably large size by etching. Referring to FIGS. 7A to 7C, how this occurs will be described in detail. FIGS. 7A to 7C illustrate a process of forming a liquid supply port 6 in a substrate 1.
First, as illustrated in FIG. 7A, the substrate 1 is prepared. The substrate 1 includes an etching mask 9 on a second surface 3 side and an etching-stop layer 11, which is made of an insulator, on a first surface 2 side.
Next, as illustrated in FIG. 7B, reactive ion etching is performed through the opening of the etching mask 9, and the liquid supply port 6 (through-hole) is formed in the substrate 1.
After passing through the substrate 1, the etching reaches the etching-stop layer 11. When etching is continued after reaching the etching-stop layer 11, as illustrated in FIG. 7C, due to the effect of electric charging of the etching-stop layer 11, a notch 14 is formed on the first surface 2 side of the substrate 1 and the etching progresses in a horizontal direction. Depending on the circumstances, the notch 14 may extend to a region in which an energy generating element and wiring for the energy generating element are formed, and the reliability of the liquid ejection head may be reduced.
Japanese Patent Laid-Open No. 2003-53979 describes a method of etching a substrate by alternatingly repeating forming of a protective film and reactive ion etching. With this method, thick protective films are formed on a bottom portion and a side surface of the hole, and only the protective film on the bottom portion is removed by etching. By etching the bottom portion while protecting the side surface with the protective film, notching that occurs at the side surface can be reduced.